This invention relates to screwdrivers and, more particularly, to self-retaining screwdrivers.
Numerous self-retaining screwdrivers have been proposed in the past, as exemplified by the devices disclosed in the following patents:
______________________________________ Patent No. Name Issue Date ______________________________________ 2,474,994 Tomalis July 5, 1949 2,646,829 Phipard July 28, 1953 4,187,892 Simmons Feb. 12, 1980 4,325,153 Finnegan Apr. 20, 1982 4,970,922 Krivec Nov. 20, 1990 5,291,811 Goss Mar. 8, 1994 ______________________________________
One well-known technique for retaining a screw on a screwdriver involves the use of a magnetized bit on the screwdriver. Another conventional technique involves the use of one or more clips or fingers mounted on the screwdriver so as to extend around the head of an attached screw and grip the underside of the screw head.
Wedging action has also been employed in a number of self-retaining screwdrivers. The blade or blades on such a screwdriver typically vary in thickness in an axial direction, whereby the driver bit wedges into an interference fit in the slotted recess of the fastener. Each slot of the fastener has to be wide enough to accommodate the extreme tip of the bit, but must be narrow enough to facilitate the wedging action.
A steep axial taper on a blade can enhance wedging action, but, with such a considerable inclination between the longitudinal axis of the driving tool and the plane of the blade performing the wedge function, there is a "throw-out" effect, that is, an axial thrust component which increases with driving torque and tends to force the driver out of the screw slot. This not only inhibits the ability to drive the screw any further, but also can ream or mar the slot and also damage the driver bit. The driver blades can be made to vary axially in thickness more gradually to minimize such effects, but such a screwdriver would only retain a screw with a correspondingly deeper slot, which makes the screw correspondingly weaker, or a correspondingly narrower slot, which would adversely affect the ease of initial entry of the driver bit into the slot.